This invention relates to a photosensitive material for electrophotography which may be repeatedly used.
In electrophotography, there are known a method wherein an electrostatic latent image is first formed on a photosensitive layer and is developed by a developer to form a toner image, and the toner image is then fixed on the photosensitive layer, and a method wherein a toner image is formed on a photosensitive layer and thus formed toner image is transferred to a transfer material for fixing. In the latter method involving the transference of a toner image, two kinds of photosensitive materials or elements are generally used, one using a vacuum evaporation film of selenium as a photosensitive layer and the other using a photosensitive layer where photoconductive powders such as zinc oxide or cadmium sulfide is dispersed in a binder of a polymeric material with high insulating property. (The latter will be hereinafter referred to as a binder-type photosensitive material for electrophotography.) In recent years, the binder-type photosensitive materials predominate over selenium photosensitive materials in a repeated transfer-type electrophotography (e.g., xerography) in which toner image is repeatedly transferred at every cycle of reproduction operations. However, the binder-type photosensitive materials are generally lower in photosensitivity than the selenium photosensitive materials. In order to enhance the photosensitivity, it is essential to use photoconductive powder with a relatively large particle size. The use of photoconductive powder with a large particle size is, however, disadvantageous in that the resulting photosensitive material or element is unsatisfactory in electric characteristics and is incable of producing an image with high quality. Especially, when such photosensitive material is applied in the repeated transfer-type electrophotography, it is impossible to effect continuous reproduction operations due to fatigue phenomenon.
For providing a binder-type photosensitive material having on its surface a photosensitive layer which has great mechanical strengths or durability, there is known a method wherein a plurality of photosensitive layers is provided on a support. The Japanese Patent Publication Nos. 8431/1969 and 33794/1974 describe photosensitive materials where two photosensitive layers are superimposed on a support, in order to improve durability of photosensitive layer. In these photosensitive materials, the upper layer has a greater ratio of binder resin to photoconductive powder than the lower layer and a resin with great mechanical strengths or durability is used as a binder for the upper layer. While these binder-type photosensitive materials serve to improve durability to some extent, they fail to improve photosensitivity at all.